HVAC Heating and cooling thermostats

Some thermostats can be used for both heating and cooling. The thermostat shown in Fig. 14-15 is such a device. The basic thermostat element has a permanently sealed, magnetic single pole double throw (SPDT) switch. The thermostat element plugs into the subbase and contains the heat anticipation, the magnetic switching, and a room temperature
thermometer. The subbase unit contains fixed cool anticipation and circuitry. This thermostat is used with 24 V AC. In this case, the thermostatic element (bimetal) does not make direct contact with the electrical circuit. Instead, the expansion of the bimetal causes the magnet to move. This, in turn, causes the switch to close or open. Figure 14-16 shows that the bimetal is not in the electrical circuit.

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